Lithography and Resist
Lithography is used to transfer an image or a pattern from a mask onto a substrate. One example use of lithography is to manufacture semiconductor devices such as integrated circuits. Since 1971, advances in lithography have allowed integrated circuit (IC) manufacturers to reduce minimum feature sizes from 10-20 microns down to 65 nanometers in 2006. This steady miniaturization has enabled improvements in IC performance and growth in the semiconductor industry.
An example optical lithography system includes a light source, a mask, a projecting optical system and a resist coated substrate. Light passed through the mask (e.g., a quartz substrate with chrome patterns on one surface) is collected by the projecting optical system to form a reduced image on the resist. The resist changes its chemical properties when exposed to the light. After developing, an identical or complementary pattern of the mask is transferred to the resist. Further processing, such as etching as one example, translates the pattern onto the substrate underneath. By repeating this technique several times using different masks, multi-layered structures (e.g., a silicon or other material based integrated circuit) can be manufactured.
Generally, resists of the type used for lithography are thin film materials that change solubility upon exposure to actinic radiation. Resists can be used as a mask to create a three dimensional structure. This process can be used to manufacture electronic devices. There are, in general, two broad families of resists: negative and positive. Negative resists become less soluble on exposure (i.e. the exposed area remains after treatment with an appropriate solvent, developer). Positive resists become more soluble after exposure (i.e. the exposed resist is removed by the developer). Within each of these two resist classifications, many different resists have been used over time. There are many chemical mechanisms that are known for both types.
Commercially available resists generally have several properties including for example:                Adequate sensitivity to the actinic radiation—Each exposure technology uses a radiation source that has a finite energy and/or intensity. The sensitivity of the resist allows the exposure system to operate at sufficient throughput.        Resolution—Each exposure technology is developed to produce features useful to manufacture devices with defined minimum features (three dimensional structures). The resist is able to resolve these features with good process latitude.        Adhesion—The resist is a thin film that is spin coated onto a device surface. The resist adheres to the surface satisfactorily to allow subsequent processing of the underling thin film.        Etch resistance—Most device processes involve the removal of selected portions of a thin film that is not protected by the resist. The resist “resists” whatever process is used to create the final, desired, pattern, viz. liquid etching, plasma etching, ion etching etc.        Low defect density—The resist preferably should not introduce additional (within reason) defects in the thin film.        The ability to use “safe processing chemicals” such as spinning solvent, developers, etc.        Ease of manufacture.        Adequate shelf life.        
Multiple chemical mechanisms have been utilized for both positive and negative resists. Some interesting negative resist mechanisms include cross-linking and molecular weight increase. For example, when a polymer is cross-linked, it becomes insoluble in common organic solvents. If the cross-linking can be induced by exposure to radiation, the material may be used as a resist to pattern thin films used in the manufacture of electronic devices. One non-limiting example is the electron beam resist COP, a copolymer of glycidyl methacrylate and ethyl acrylate. Cross-linking occurs through the epoxy moiety. Another negative resist is based on crosslinking of cyclized poly(cis-isoprene) with bis(arylazide). In addition, solubility of a polymer is generally related to the molecular weight of the polymer. As the molecular weight increases, the solubility decreases. Poly(p-hydroxystyrene) (PHOST), when formulated with bis(arylazide), undergoes a radiation induced molecular weight increase, resulting in decreased solubility. The material can be made sensitive to a wide range of radiation wavelengths by modifying the structure of the bis(arylazide).
Example positive resist mechanisms include mechanisms such as:                Chain scission—Most polymers crosslink as a result of irradiation; however, a few undergo chain scission and a reduction in molecular weight. The lower molecular weight allows the exposed polymer to be selectively dissolved in an appropriate solvent (developer). Poly(methyl methacrylate) (PMMA) is a well known polymer that undergoes chain scission and has been widely used as an electron beam resist. The sensitivity of PMMA is to low to be used in manufacture. Another family of polymers, poly(olefin sulfones) exhibit˜10× greater sensitivity than PMMA and poly(butene-sulfone) has been used for a long time as an electron beam resist in the manufacture of photomasks.        Chemical amplification—Very sensitive positive resist based on chemical amplification have been developed. Example processes typically involve photo-generation of an acidic species (some base catalyzed systems have been described) that catalyzes many subsequent reactions such as de-blocking of a protective groups that are chemically bound to a matrix polymer. One such system is based on a matrix resin, poly(4-t-butoxycarbonylstyrene) (TBS) and arylsulfonium or iodonium salts. Radiation is used to generate an acid which in turn removes the t-butoxycarbonyl resulting in the base soluble poly(vinylalcohol). One acid group causes up to several hundred de-protection events, thus amplifying the desired reaction. These materials and derivatives thereof are in wide spread use as the resist of choice in deep—UV (248 nm & 193 nm) lithography.        
All resists used in the current production are linear resist, they can not generate patterns smaller than the diffraction limit allows. A non-linear resist combined with double or multiple patterning is needed to created sub-diffraction limit patterns.
Two-Photon Resist and Multi-Photon Resist
In a quantum system with two levels, initial level E1, and final level E2, a photon having energy E2-E1 can be absorbed, promoting an electron from E1 to E2, in a one photon absorption process. Also, a less likely process, called two-photon absorption, can occur. In this process, two photons with energy (E2-E1)/2 can be absorbed simultaneously. A two-photon absorption process has smaller probability than a one-photon process because it requires a simultaneous presence of two photons at same location. Likewise, three-photon, four-photon, and multi-photon can be absorbed with decreasing probability.
In a two-photon absorption
                                          ⅆ            I                                ⅆ            x                          =                              -            β                    ⁢                                          ⁢                      I            2                                              (        1        )            where I is the intensity of the beam and β is defined as the two-photon absorption coefficient to parallel the one photon, or linear, absorption regime:
                                          ⅆ            I                                ⅆ            x                          =                              -            α                    ⁢                                          ⁢          I                                    (        2        )            where a is the one photon absorption coefficient.
The two-photon absorption cross section is defined through the absorption rate:R=δI2  (3)Note here I is the number density of photon (number of photons per second per unit area) and δ the two-photon absorption cross section.
Wu et al. proposed a two-photon resist used in optical lithography. See E. S. Wu, J. H. Strickler, W. R. Harrell, and W. Webb, Proc. SPIE 1674, 776(1992). In a two-photon resist, the photo sensitizer in the resist will only be exposed through a two photon absorption process. Due to the quadratic dependence to the intensity, the two-photon resist is capable of creating sharpened features in the resist. As evidenced by the normalized exposure profile shown by in FIG. 1. A standard testing pattern in lithography is lines and spaces created by two interference plane waves. At the diffraction limit, the light intensity distribution at the resist can be expressed as:
                    I        =                  1          +                      cos            ⁡                          (                                                                    4                    ⁢                    π                    ⁢                                                                                  ⁢                    NA                                    λ                                ⁢                x                            )                                                          (        4        )            where NA is the numerical aperture of the optical system and X is the wavelength of light.
In FIG. 1, an aerial pattern is transformed into a sharper resist profile (P2) compared to a linear resist (P1). P1, P1.5, P2 and P4 are 1, 1.5, 2 and 4 photon absorption profile, respectively. Combined with double patterning or multi-exposure, the two-photon resist is capable of producing sub-diffraction limit image and is a promising technique to extend optical lithography beyond its current limit. See e.g., Ch. J. Schwarz, A. V. V. Nampoothiri, J. C. Jasapara, W. Rudolph, and S. R. J. Brueck, J. Vac. Sci. & Tech. B 19 (6): 2362-2365 (2001). FIG. 2 demonstrates how a two-photon resist enables double patterning. With two exposures (P1A and P1B) shifted by a quarter of the spatial period will result in a uniform exposure (PF1) in a linear resist, as shown in FIG. 2a, a linear resist sums up the two exposures and results in a constant exposure, all contrast is lost. A two photon resist is a non-linear resist. A nonlinear resist has a nonlinear response rate to either exposure intensity or time, or both. In an ideal two-photon resist, the two exposures (P2A, P2B) will result in an exposure profile (PF2) with doubled spatial frequency, as shown in FIG. 2b. If the spatial frequency of the light pattern of each exposure is at the diffraction limit then this double patterning process enables sub-diffraction limit lithography.
In fact, similar to the above argument a multi-photon absorption process can be used to produce a multi-photon resist. In a multi-photon process, the absorption rate, R:R=δIP  (5)where P equals to the number of photons involved in one absorption event. Multi-photon resist is capable of achieving even higher resolution, as shown in FIG. 2 for an example of P=4 (P4).
Further, in equation (5), the resolution will still be improved even if 1<P<2. As shown in FIG. 1. for P=1.5 (P1.5).
Current two-photon resists, however, are mainly used to create 3-D patterns, not in planary pattern creation. The main reason is the extremely high light intensity involved. The conventional two-photon absorption process is after all a second order process. It requires absolute coincidence of two photons on the absorbing molecule. The absorption cross-section is extremely small, ˜10−50 cm4 S. See E. S. Wu, J. H. Strickler, W. R. Harrell, and W. Webb, Proc. SPIE 1674, 776 (1992). To achieve a practical intensity, a pico-second or femto-second laser has to be used. The DUV lasers used in current lithography industry has pulse width ˜10 ns. We describe a new type of two-photon resist based on a mechanism other than the traditional two-photon absorption. The exposure in this resist may have a quadratic or higher order dependence on the light intensity yet it may not involve a traditional two-photon absorption, therefore we refer to it as I2 resist. A two-photon resist, by our definition, is a special case of I2 resist.